6. Marvin Gaye and Tammi Terrell, "Ain't No Mountain High Enough"
Real-life couple Nick Ashford and Valerie Simpson knew they had something special on their hands when they wrote "Ain't No Mountain High Enough" in 1966. They even declined an offer from Dusty Springfield to record the track since they thought it was get them access to the hallowed halls of Motown. The plan worked and it became the first of many duets recorded by Marvin Gaye and Tammi Terrell. The duo had a brief run of amazing hits, but Terrell was sidelined by a brain cancer diagnosis that took her life in 1970. She was only 24.